The present invention relates to a rotary speed detector and, in particular, to a rotary speed detector which calculates revolutions per minute (rpm) of an electric motor of a vehicle.
A conventional rotary speed detector includes a resolver, a transforming means, a measuring means and a calculating means. The resolver is fixed to an output shaft of an electric motor and outputs analog signals which represent rotational angle data of the shaft, that is, data pertaining to the degree of rotation with respect to a known baseline. The transforming means transforms the analog signals into angle pulse signals. The angle pulse signal is equal to or occurs at some fraction of a revolution. For example, the angle pulse signal provides that one cycle should be equal to one-quarter revolution of the shaft. The measuring means measures each output cycle, from the start of an angle pulse signal until the end of the angle pulse signal. The calculating means calculates the revolving speed of the electric motor from the cycle period.
However, in using the detector described above, as the speed of the electric motor becomes lower, the measuring period for each cycle becomes longer. Therefore, calculating the rotary speed requires a long time, when the revolutions per minute (rpm) of the electric motor are low.
In addition, the measuring means further includes a counting pulse source for measuring the cycle period. The counting pulse source outputs counting pulses at regular intervals. The output of the counting pulses is without regard to the angle pulse signals. As a result, the accuracy of the measuring cycle period is not constant between a low revolving speed and a high revolving speed.
The present invention provides a rotary speed detector without the foregoing drawbacks.
In accordance with the present invention, a rotary speed detector comprise a rotating angle sensor outputting digital signals which constitute rotating angle data of a rotary member, an angle variation calculating means for calculating an angle variation with respect to a predetermined interval of time and a rotary speed calculating means for calculating rotary speeds from the angle variation.